The present invention resides in a bulk thermal energy storage device. More particularly, the invention resides in a bulk thermal energy storage device comprising a container or tank for storing a hydrated phase change material (PCM) which is subject to repeated freezing and melting cycles during which latent thermal energy in the composition is released or, alternatively, thermal energy is absorbed by the composition. At least one heat exchange device is positioned within the PCM inside of the bulk container in a substantially vertical position. The heat exchange device has a flow passage or channel therein for flow of a heat exchange fluid through the channel. The heat exchange device is positioned in the container in a spaced relationship to the interior walls of the container to permit convection currents to flow through the spaces provided between the interior wall surfaces off the bulk container and the exterior wall surfaces of the heat exchange device during the melting cycle of the PCM. At least one screen is positioned in the bulk container extending substantially horizontally and at a distance from the bottom of the container and adjacent to the heat exchange device for supporting solid crystalline particles of the PCM which are present at the onset of the melting cycle of the PCM. The supporting screen prevents any solid crystalline particles from settling or falling to the bottom of the container, during the onset of the melting cycle, where the crystalline particles are no longer sufficiently exposed to a direct contact with the convection currents of the melted PCM to melt such solid crystalline particles. The screen supports the solid particles in a position adjacent to the heat exchange device and above the container bottom such that the convection currents of the molten PCM can contact the solid particles until the solid particles have absorbed sufficient thermal energy from the molten PCM to undergo a phase change from a solid to a molten state.